a. Smo is activated when it combines stoichiometrically with a small molecule (sm, red) that changes its conformation. The small molecule must be actively transported across the cell membrane by Ptc1, or is prevented from exiting the cell membrane by Ptc1, which acts catalytically. Active forms are in color, inactive in gray. b. A time series for activated Smo concentration is shown when inactive Smo combines stoichiometrically with a small molecule activator. This is a second order reaction. Analytic solutions for first, second and third order kinetics are also shown. Rules for the model is simulated are described in the text.